A River Prone to Flooding, and Misunderstanding

Tuesday

Over the last week, the Red River in North Dakota has emerged as perhaps even more maddeningly complex, and in some ways harder to predict, than before.

FARGO, N.D. — Predicting the weather has always been at least in part a gambler’s game — a matter of odds and percentages.

But over the last week, as the Red River in North Dakota has surged to potentially catastrophic flood levels, setting off waves of anxiety from here to Washington, forecasters seem to have been betting mostly on the wrong horse.

The flood surge rose much faster than expected in Fargo, the state’s largest city, then peaked sooner and at a lower level than forecast — to the city’s great relief and gratitude. In the last two days — surprise again — it has gone down more rapidly than foreseen.

But the uncertainty has taken a toll.

“It really stresses the city’s system,” said Donald P. Schwert, a professor of geology at North Dakota State University in Fargo, who has been a consultant on landslide and erosion issues to Cass County, which includes Fargo. “The city builds up temporary dikes on a forecast, then a new forecast comes and the city has to respond to that, and on it goes.”

Scientists say they have learned a tremendous amount about the Red River since its last major flood in 1997, using sophisticated modeling systems developed in the wake of disasters up and down the river that year.

But to the chagrin and frustration of emergency workers, one of the biggest lessons from all the new data is that the Red River — obscure to many Americans, but beloved in the world of river hydrology — has emerged as perhaps even more maddeningly complex, and thus in some ways harder to predict, than before.

“It’s like anything else in life — the more you know, the more you know you don’t know,” said Scott Dummer, the hydrologist-in-charge at the National Weather Service’s North Central River Forecast Center.

Mr. Dummer (pronounced DUE-mer) said the Red River, though fairly modest compared with some more famous rivers, was devilishly hard to predict, partly because of its shallow channel. The Colorado River has been carving out the Grand Canyon for millions of years. The Red, by contrast, dates back to perhaps only a few thousand years before the Pyramids. That means it has not had that long to cut deep channels that can contain water during floods.

On top of that, the river flows very slowly across a pancake-flat landscape. Imagine raising an eight-foot-long sheet of plywood just enough to slip a single sheet of paper under the raised end. The resulting minuscule tilt of the board represents the average slope of the Red River’s bed.

What that means is that the river, when it goes awry during a flood, spills every which way across the countryside. This makes predictions of flood levels contingent on thousands of data points, not just depth gauges here and there.

In the Weather Service’s defense, Mr. Dummer said the long-term predictions of this year’s flood — the first warnings went out in December — were right on the money, and justified the expense and work involved in the new computer models, which rely on 58 years of river data.

Other wrinkles of the river’s drainage basin, though, are just now being explored, like the odd legacy of homesteading. The land grant system of the 1800’s divided much of the nation into square-mile sections of 640 acres — a pattern still prevalent on the Great Plains, where many roads follow with geometric, if not downright boring, exactitude the old ruler-straight division lines.

Now comes the Red River question: How much water does each square hold? Nobody knows the exact amount, said Aaron W. Buesing, a hydraulic engineer with the United States Army Corps of Engineers in St. Paul, but the next round of computer models aims to provide an answer.

Mr. Buesing said he thought that grid storage might explain why some flood surge predictions were off. The river’s quick rise, accompanied by a cold snap, may have trapped enough water in the grids to keep the worst predictions from materializing, he said.

Then there’s Canada to worry about. Squashed by glaciers for thousands of years, it has been slowly recovering from the compression. For the north-flowing Red River, that means its downhill slope, already barely perceptible, is getting even less pronounced with each passing year, adding to its complexity, and its propensity to flood.

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